The invention relates to a method and apparatus for covering a metal roll core with a polymeric material such as a high performance thermoplastic material. However, the present invention is not limited to the use of high performance thermoplastic materials and contemplates the use of other extrudable elastomers such as rubbers or processable urethanes, and high viscosity thermoset resins such as polyurethanes and epoxies in general. These covered rolls are utilized in many applications including an application known as calendering. Calendering is the act of pressing a material, e.g., cloth, rubber or paper, between rollers or plates in order to smooth or glaze or to thin into sheets. The covered rolls that are discussed in this application are also known as calender rolls, soft-nip calender rolls and supercalender rolls and are often utilized in industrial environments such as paper mills. However, the invention described herein is not limited to covered rolls known by these names or utilized in these environments.
In a typical paper mill, large numbers of rolls are used not only for transporting the web sheet which becomes paper, but also for calendering or processing the web sheet into a specific grade of paper. The finished paper product must possess certain quality characteristics such as a high degree of caliper uniformity, bulk uniformity, smoothness, gloss and printability. In order to achieve these quality characteristics, it is necessary that the calender roll be precisely manufactured utilizing materials that can withstand severe conditions during paper processing.
For example, when used for transporting the web sheet, it is essential that these covered rolls provide traction to enable the transport of the paper during processing. Additionally, these rolls must be wear and corrosion resistant. During use for calendering, these covered rolls are subjected to high dynamic stress, heat, speed, abrasion and impact and therefore must be fabricated to withstand these elements. In order to function properly for these specific uses, the covered rolls must have an appropriate surface hardness based upon the intended application for the covered roll and also have a high thermal resistance to withstand high temperatures and pressures in the environments in which they are employed. Regardless of their application, these covered rolls are precision elements of the systems in which they are utilized and therefore must be precisely manufactured to achieve balance, specific size and shape specifications, surface characteristics and tight tolerances. The covered rolls have similar transporting and calendering functions in the textile industry as well as in facilities where magnetic tape is manufactured.
Conventional prior art calender rolls comprise a metal cylinder to which either a cotton-filler or a thermoset composite layer (or layers) is added to preclude metal-to-web-to-metal contact at the nip between the calender rolls during the calendering operation. Though cotton-filled roll covers have been used for a long time there are several drawbacks associated with their use such as the need for frequent regrinding. Moreover, cotton filler material is not sufficiently tough to withstand the high stress and high temperatures associated with demanding applications such as in modem paper fabrication. Paper mills must frequently regrind and replace cotton-filled roll covers, even when they perform well. This results in significant production down-time and higher costs associated with keeping replacement rolls in inventory.
Over the last two or three decades, synthetic composite roll covers have been developed to resolve many of the problems associated with cotton-filled roll covers. Most of these synthetic composite roll covers use some form of thermoset resin such as epoxy, rubber or polyurethane among others, as a base material which is combined with some form of reinforcement material to improve strength.
As an example, a synthetic composite roll cover is formed of a single layer of a reinforcement fiber mat that is impregnated with a thermoset epoxy which is then cured. The surface of the cured single layer is then machined to a smooth finish in accordance with customer specifications.
Alternatively, rather than machining the cured single layer to a smooth finish, an additional layer of reinforcement fiber mat may be added over the cured single layer, the additional layer being impregnated with an epoxy which is then cured to form a top layer. The surface of the cured top layer, which provides the outer surface of the roll is then machined to a smooth finish in accordance with customer specifications. The single layer, which forms an under layer, provides a transitional element between the metal core and the top layer to assist in establishing an effective bond and stress distribution between the two layers of the covered roll. Alternatively, additional layers could be added.
In practice, a layer of the synthetic composite cover is added to the roll core by unspooling a strip of dry reinforcement fiber mat, several inches in width, from a reel and conveying the unspooled strip through an epoxy bath. The roll core is oriented horizontally and rotated to wind the epoxy impregnated strip onto the roll core in a back and forth fashion to form a helix. The epoxy is then allowed to cure to form a layer which is then machined to a smooth finish.
The use of these synthetic composite roll covers has increased dramatically in the last ten years because of their superior performance characteristics over conventional cotton-filled roll covers. The acceptance and usage of synthetic composite roll covers in the paper industry has resulted in the beginning of the demise of the cotton-filled roll cover. Notwithstanding their superiority over cotton-filled roll covers, synthetic composites such as thermosetting epoxies also suffer from several drawbacks. For example, to formulate a synthetic composite roll cover having certain desirable properties such as high toughness, high temperature capability (glass transition temperature (Tg)), it is usually necessary to employ a higher concentration of reinforcement fibers. Increasing the concentration of reinforcement fibers in this manner can result in the emergence of other less desirable properties such as unacceptable surface finish, easier delamination, greater brittleness, and poor bonding between the cover and the metal outer surface of the roll core. Roll manufacturers struggle to optimize these conflicting properties to achieve a superior roll cover. Failure and inconsistent performance of the synthetic composite roll covers in the field have been and continue to be a common problem. Even with recent advances in resin chemistry, synthetic composite roll covers are best performing at operating conditions wherein the maximum operating temperature does not considerably exceed 250xc2x0 F., the maximum nip pressure does not considerably exceed 10,000 p.s.i., and wherein the surface roughness of the cover is considerably less than 10 Ra micro-inches.
The method and apparatus of the present invention enables the fabrication of rolls covered with synthetic composite materials such as those discussed above. The method and apparatus of the present invention also enables the fabrication of rolls covered with high performance or engineered thermoplastic materials. Some thermoplastic materials have a number of highly desirable properties making them superior to synthetic composite materials being utilized today in the fabrication of covered roll cores. This includes a higher glass transition temperature, a more suitable Young""s modulus for many applications, a higher tensile strength, greater smoothness, a higher impact strength, more uniform surface finish and more homogenous physical and thermal properties. Thus, a roll core covered with a high performance thermoplastic material will achieve superior performance characteristics than one covered with a synthetic composite material such as thermosetting epoxy resin.
Accordingly, it is a general object of this invention to provide a method and apparatus for covering a roll core with an outer layer material that overcomes the disadvantages of prior art cover materials.
It is a further object of this invention to provide a method and apparatus for covering a roll core with a thermoplastic material as the outer layer.
It is a further object of this invention to provide a method and apparatus for covering a roll core with a synthetic composite material as the outer layer.
It is a further object of this invention to provide a method and apparatus for covering a roll core with a thermoplastic material which prevents sagging of the thermoplastic material prior to solidifying.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has minimal residual stresses.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover effectively adheres to the metal outer surface of the roll core it covers.
It is a further object of this invention to provide a method and apparatus for covering a roll core that results in a polymeric covering having a high tensile strength.
It is a further object of this invention to provide a method and apparatus for covering a roll core that results in a polymeric covering having a more suitable Young""s modulus.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high glass transition temperature.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high durability and long lifespan.
It is a further object of this invention to provide a method and apparatus for covering a roll core that is less expensive than prior art methods and devices.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the roll core is oriented vertically within the apparatus rather than horizontally during the application of the cover.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has an outer surface that is smoother than prior art covers formed of thermosetting materials such as epoxy resins.
It is a further object of this invention to provide a method and apparatus for covering a roll core that results in a cover that performs consistently under extremely high pressures, high heating conditions and high speed conditions.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the cover has a high compression strength.
It is a further object of this invention to provide a method and apparatus for covering a roll core wherein the covering has a high impact strength.
It is a further object of this invention to provide a method for covering a roll core that is shorter in fabrication time than the prior art methods.
These and other objects of this invention are achieved by providing a method and apparatus for covering a roll core with a polymeric material, preferably a high performance thermoplastic material. The method includes first providing a cylindrical roll core having two ends, a length, and an outer surface. A spacer ring assembly is attached proximate one end of the roll core, the spacer ring assembly having a circumference greater than that of the roll core. After being preheated to a desired temperature, the roll core is then placed within an apparatus in a substantially vertical orientation and held therein by opposed universal chucks. A length of mold tape is helically wound over the length of the roll core in a spaced-apart relationship therewith to define an application zone between the mold tape and the roll core outer surface. An extrudate formed of a polymeric material, preferably a high performance thermoplastic material having a continuous profile, is extruded within the application zone and helically wound over the roll core outer surface so that the roll core is covered with the extrudate.
Localized heating is applied to the roll core surface just prior to the application of the extrudate to improve bonding to the roll core surface. The mold tape dispenser, induction heating device, and the extruders are all located on a turntable that rotates concentrically around the roll core while the roll core remains non-rotational and is lowered from an elevated position. The mold tape acts as a supportive form to prevent sagging of the extruded material prior to solidifying. The wound extruded material is allowed to solidify in a temperature controlled manner to be hardened to form a continuous layer over the length of the roll core. Finally, the mold tape is removed and the covered roll is machined to a predetermined roughness. The resulting covered roll fabricated in accordance with this method has minimal residual stresses and a has a higher tensile strength, compression strength and impact strength. The resulting covered roll also has a higher glass transition temperature, a more suitable Young""s modulus, greater durability and a longer lifespan than prior art covered rolls. The covered roll fabricated in accordance with this method also performs consistently under extremely high pressures, high heating conditions and high speed conditions.